Ignition systems are well known in which the ignition coil, the distributor, the electronic switching circuit as well as the control circuit for timing the spark are electrically connected to each other and mounted in the engine compartment of the motor vehicle. The control circuit in particular is subjected to large temperature variations and shock and vibration, as well as electric and electromagnetic noise. In particular, when electronic apparatus is used to determine the ignition timing, such noise can have very deleterious effects. It is impossible to adequately shield the computing circuits against the electrical and electromagnetic interference which results from the sparks at the spark plugs.